The Tide in Filey Bay
The Ups and Downs and Ins and Outs at Filey
The ups, downs and sideways movement of the sea can be a complex subject and much has been written about tides and tidal streams in navigation and sailing books.
To have some understanding about what is going on in Filey Bay you have to consider what is affecting the water.
The main thing is gravity: the attraction of the Sun and Moon on the Earth’s surface. The Moon is, of course, much smaller than the Sun but being so close to the Earth it has an attractive force over twice that of the Sun. The Moon causes the water to move up towards it causing a high tide and at the other side of the Earth the water is being thrown out by centrifugal force as the Earth spins on its axis. As the Earth spins, these high waters pass around the circumference causing high and low waters. The Earth turns on its axis once every 24 hours, so this gives us two high waters and two low waters every day. Well, not exactly, because the Moon is also travelling round the Earth every 27½ days. This means it has moved in relation to the Earth giving us high water every 12 hours 25 minutes. Thus, the high water time moves forward by 50 minutes every day, but this is only a rough guide as there are other factors to take into account.
Talking about rough guides, how about this: if you see that it is high water now, then in one week’s time it will be low water at about this time and in another week it will be high water again at about this time.
When we have a full or new Moon it means that the Sun, Moon and Earth are all in a line and the gravitational attractions are all working together to pull up as much water as possible. This causes the sea to reach a maximum height called Spring Tide. There is a delay of a couple of days after a full or new Moon before the movement of water catches up with the gravitational pull. Massive amounts of water are being constantly pulled around our Earth every day but land gets in the way which alters the times and intervals of high water in different areas.
When the Moon is between full and new it is in its first or last quarter also known as half moon and has less pull on the water so we get a lower tide called Neap Tide.
The orbits of the Earth and the Moon are not circular and their angle of orbit (called declination) change. Therefore, when the Moon is closest to the Earth (perigee), maximum pull is exerted and we get very high spring tides which occur near the equinoxes on 21st March and 23rd September. Near the solstices on 21st June and 22nd December the high tides are smaller than normal.
Now that you think that you understand it, there is more. The fancy term is meteorological effects. High and low atmospheric pressure on the sea can vary the predicted levels. The one to watch out for is low pressure, this takes the pressure off and the sea level pops up by as much as 30cm or a foot. If this is combined with a strong on-shore wind and rough seas you can add even more. If these waves happen on a spring tide near an equinox then serious damage can be caused.
When you get a local tide table try to get one that shows both the high and low water heights and times and has been adjusted to allow for the hour of British summer time. If the tide table is for Scarborough add 13 minutes for Filey time. Heights of tide are given in these tables from Chart Datum; this is the level of lowest astronomical tide, which can fall under average meteorological conditions, and any combination of astronomical conditions.
At Filey, Spring tides can be over 6 metres and as low as 1 metre, giving a range of 5 metres. This means that when it is low water you have sand well out past the metal pole off Cobble Landing and at high tide the water is well up the slipway. So, if you want to bring your boat in onto sand do it well before high water.
The rule of twelfths is a method of predicting the height of the tide (and the flow of water) between high and low water. It can be applied to Filey because the rise or fall of tide is about 6 hours and is symmetrical
Slack water 1st hour rises or falls by 1 twelfth 2nd hour rises or falls by 2 twelfths 3rd hour rises or falls by 3 twelfths 4th hour rises or falls by 3 twelfths 5th hour rises or falls by 2 twelfths 6th hour rises or falls by 1 twelfth Total 12
It can be seen then, that in the middle hours between high water and low water the tide goes out by half of its range. Apply this to Filey Bay when the tide is going out (ebbing) and you can see how fast the water must move in these middle hours to empty half the bay. Let us say for ease of calculation that the distance from Filey Brigg to Flamborough Head is 10 miles, and the distance from the Brigg to the shore is a mile, and the drop of water between tides is 5 metres. So, between the tides in this area there is a movement of 128 million tonnes of seawater. Therefore in the middle two hours between tides 64 million tonnes moves in or out. Bear this in mind in light winds or you could be swept out of the Bay, around the Brigg and on your way to Scarborough!
While we are on the subject of Scarborough, when sails to Scarborough are organised they try to choose a day when the high tide is 6 or 7 am, low tide at Scarborough is then at lunch time. The reason for this is when you set off from Filey. The ebb tide pushes or carries you on the magic carpet of moving water towards the north in the direction of Scarborough. If the wind drops then get inshore before it carries you past Scarborough! Have lunch at Scarborough and then be carried by the flood tide (coming in) back towards Filey. So, remember – out North, in South.
When the tidal stream flows up and down our coast beware because the time of the change in its direction from north to south or south to north is not the same as the time for high or low water. The direction can change up to an hour and a half after high or low water. It’s a bit like a river, just because it is flowing through the river does not mean that the water is rising on the bank side.
When you get near to Filey Brigg the tidal stream flowing over this large underwater obstruction causes the water to rise up, speed up (up to 5 knots) and make the water surface very turbulent, these are overfalls. When the direction of the tidal stream and a strong wind are in different directions you have a wind over tide situation. This can push up large waves and create wonderful eye-catching plumes of water as they crash into and over the Brigg, when hopefully you are watching from the clubhouse!
Note that wind directions are given in the direction the wind is coming from – a northerly wind is blowing from the north. However tidal stream directions are given in the direction they are going to - 90° is going towards the east.
There is another way the water can move that can affect small boats and this is surface drift. This is caused when the surface water moves because of a constant or strong wind.
When you are sailing towards the Bell Buoy look for a crab pot flag that you can keep in line with. This will help you judge how much the tidal stream is pushing you off course and which is the safe side on which to pass the Buoy. You do not want the tidal stream to carry your boat onto that large chunk of steel. Look at the base of the Buoy and you will see a tail of water coming from it giving you the direction of the tidal stream.
The shape of Filey Bay means that the tidal streams are bent causing the water to move with a circular motion. This motion tends to be anti-clockwise when the tide is coming in going towards the south, and clockwise when going out towards the north, but there are also counter currents (eddies).
One of the main attractions of Filey Bay is that it continually changes and this can enhance your enjoyment of sailing as you respond to it.
David St. Claire Bruce
